Gas-filled display devices have been known and used for a considerable length of time. One known type of device comprises a small gas-filled glass envelope having anode and cathode electrodes sealed in the envelope. Another type of device is known as a display panel and comprises a flat structure which has a large number of gas-filled cells arrayed in rows and columns and having a row or X conductor for each row of cells and a column or Y conductor for each column of cells, with one cell being located at each intersection of a row and column electrode. These prior art devices are caused to turn on, and strike a glow discharge, by means of applied D.C. or A.C. voltages, but, when the applied voltages are removed, the devices generally turn off and do not glow.
There are some relatively complex display devices having auxiliary electrodes in which the ON state and glow discharge can be sustained by the application of a potential other than the initial firing potential. Usually, in the operation of these devices, a bias voltage is applied which is below the firing voltage but above the extinction voltage, and then a triggering or signal voltage is added so that the total applied voltage is equal to, or exceeds, the firing voltage, and the device is turned on. Thereafter, when the triggering pulse terminates, the cell is maintained ON by the bias voltage.
There are also relatively complex circuits which have been devised to sustain glow discharge by storing or continually regenerating the applied signal information. In these circuits, it has been considered necessary to provide a current-limiting element or impedance, such as a resistor or capacitor, in series with each individual display device or cell to control and limit the current therethrough to a safe level while glow discharge was sustained. It has also always been necessary to provide a resistor in series with each cell of a multi-cell panel to obtain uniformity in light output and to limit current flow in the individual cells.
It can be seen that panel display devices which include tiny light-producing cells might have tens or hundreds of thousands of such cells, and individual resistors for these cells would represent a considerable parts cost. In addition, the operation of assembling the resistors with a panel would represent a considerable labor cost and an added complex manufacturing operation.
In addition to the need in the prior art for uniformity in the cells of a panel, it is also necessary that the circuits for operating multi-cell panels themselves be stable and uniform in the circuit components which generate the voltages which drive the cells.
Another type of multi-cell gaseous display panel is currently being developed at the University of Illinois. In this panel, the row and column conductors are separated from the gas cells by thin sheets of glass, so that the conductors are only capacitively coupled to the cells. An A.C. signal applied to all of the conductors imparts a memory characteristic to the matrix, by virtue of stored charges on the glass walls of the cells, and triggering to an ON condition is achieved by trigger pulses applied to selected cells at a precise time during the A.C. sustaining signal.
This matrix disadvantageously requires a sustaining A.C. signal in the order of 1200 volts peak-to-peak, a triggering voltage across a cell of about 400 volts precisely synchronized with the A.C. signal to occur at a particular phase thereof. This method of operation requires considerable uniformity in panel cells, and the required uniformity is difficult to achieve.